A Wood’s lamp uses ultraviolet A (UVA) light, emitting wavelengths between 320 and 400 nanometers with a peak output at 365 nm. This places it in the “long-wave” ultraviolet range, just beyond the violet edge of visible light. The lamp is sometimes called a “black light” because its output is invisible to the naked eye, yet it causes certain substances in the skin to glow, or fluoresce, in distinctive colors.
How the Filter Works
What makes a Wood’s lamp different from an ordinary UV source is its specialized glass filter. Traditional Wood’s glass is a mixture of barium-sodium-silicate glass containing 9% nickel oxide. This combination blocks most visible light while allowing UVA wavelengths to pass through. The result is a beam that looks dim purple to the human eye but bathes the skin in invisible ultraviolet energy. When that energy hits certain bacteria, fungi, or pigment changes, those substances absorb the UV and re-emit it as visible colored light, a process called fluorescence.
Traditional Tubes vs. Modern LEDs
The original Wood’s lamps relied on mercury vapor tubes fitted with the nickel-oxide glass filter. These tubes are effective but fragile, bulky, and require a warm-up period before they reach stable output. In recent years, UV LED-based lamps have become widely available. These newer devices are compact, more durable, and less expensive. They produce a narrower band of UV light centered on the same 365 nm peak, which can actually improve contrast when examining the skin. LED versions are particularly useful in field settings and developing countries, where traditional glass tubes are impractical.
What Different Colors Mean
The diagnostic power of a Wood’s lamp comes from the specific colors different organisms and skin conditions produce under UVA light. Each glow acts like a visual fingerprint.
- Coral pink: A hallmark of erythrasma, a bacterial skin infection in body folds. The bacteria responsible produce a compound called coproporphyrin III, which fluoresces that distinctive coral-pink color.
- Blue-green: Certain fungal scalp infections (tinea capitis) caused by Microsporum species glow blue-green under the lamp.
- Dull blue: Another fungal species, Trichophyton schoenleinii, produces a subtler blue fluorescence.
- Yellowish or orange: The yeast responsible for pityriasis versicolor, a common scaly rash on the chest and back, emits a yellow-orange glow when active.
- Green: Pseudomonas bacteria, sometimes found in hot tub folliculitis or wound infections, fluoresce green.
- Orange-red: Acne-related bacteria living in hair follicles produce an orange-red fluorescence.
- Red-pink: Porphyria, a group of metabolic disorders, causes red-pink fluorescence of the skin or even the teeth, depending on the type.
- Bright blue-white: Hypopigmented (lighter) skin patches appear with sharply defined borders and glow blue-white, making conditions like vitiligo far easier to see than under normal room light.
How Pigmentation Disorders Show Up
One of the most common clinical uses of a Wood’s lamp is evaluating pigment changes that are hard to see on their own. In vitiligo, patches of skin that have lost their pigment-producing cells fluoresce a striking bright blue-white, making even subtle or early-stage patches visible. This is especially helpful on fair skin, where the contrast between normal and affected areas may be minimal under regular lighting. The lamp also helps distinguish whether excess pigment sits in the upper layers of the skin (epidermal) or deeper layers (dermal), which influences treatment decisions for conditions like melasma.
Practical Details of the Exam
During a Wood’s lamp examination, the room is darkened so the fluorescence is easier to spot. The lamp is held about 5 inches from the skin and passed over the area of interest for a few seconds. The exam itself is painless and quick. You may be asked to wear protective goggles or close your eyes, because repeated UV exposure at close range can damage the eyes over time. The examiner typically wears goggles as well.
Certain topical products can interfere with results. Soaps, makeup, deodorants, and some moisturizers may fluoresce on their own or mask the natural fluorescence of an infection. For the most accurate reading, the skin being examined should be clean and free of these products.